proxy,ui: add performance monitoring with Prometheus metrics (#743)

Add a comprehensive performance monitoring system that collects CPU, memory, swap, load average, network IO, and GPU stats. Provides both a REST API for the UI and a Prometheus /metrics endpoint.

Backend changes:
- New internal/perf package with configurable interval-based stats collection
- GPU monitoring via LACT (Unix socket) and nvidia-smi fallback on Linux
- Ring buffer (internal/ring) for time-series stat storage
- Prometheus /metrics endpoint with all system and GPU metrics
- Moved LogMonitor to internal/logmon package
- New PerformanceConfig for hot-reloadable monitoring settings
- REST /api/performance endpoint replacing SSE streaming

UI changes:
- New Performance page with real-time charts for CPU, memory, GPU, and network
- Reusable PerformanceChart component
- LLAMA_SWAP_URL environment variable support
- Improved capture dialog display

Other:
- Example Grafana dashboard for Prometheus metrics
- monitor-test standalone binary
- Config schema and example updates

fixes #596

ui-svelte/src/App.svelte

@@ -5,6 +5,7 @@
   import LogViewer from "./routes/LogViewer.svelte";
   import Models from "./routes/Models.svelte";
   import Activity from "./routes/Activity.svelte";
+  import Performance from "./routes/Performance.svelte";
   import Playground from "./routes/Playground.svelte";
   import PlaygroundStub from "./routes/PlaygroundStub.svelte";
   import { enableAPIEvents } from "./stores/api";
@@ -16,6 +17,7 @@
     "/models": Models,
     "/logs": LogViewer,
     "/activity": Activity,
+    "/performance": Performance,
     "*": PlaygroundStub,
   };